inertial reference frame

An inertial reference frame is a reference frame or coordinate system in which there are no accelerations, only zero or uniform motion in a straight line – in other words, in which the first of Newton's laws of motion is valid. According to the special theory of relativity, it is impossible to distinguish between such frames by means of any internal measurement. For example, no measurements made inside a spaceship traveling at high speed (even close to the speed of light) relative to some locally agreed stationary frame, such as the Earth, can show different results from similar measurements made when the ship is at rest relative to the local stationary frame.